Lubricating oil compositions containing sulfur corrosive to silver



United States Patent LUBRICATING OIL COMPOSITIONS CONTAINING SULFUR CORROSIV E TO SILVER Warren Lowe, Berkeley, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Application December 5, 1951, Serial No. 260,112

5 Claims. (Cl. 25247) The present invention relates to lubricating oil compositions containing sulfur which is corrosive to silver and like metals, which corrosion is inhibited by the presence of 2-thio-thiazoles (e. g., 2-mercaptothiazole) and thiadiazolyl dithiocarbamates (e. g. 1,3,4-thiadiazolyl-2,5- bis(dialkyl dithiocarbamate) For the most part, internal combustion engines are operated at high speeds and at eleavted temperatures. At high speeds and elevated temperatures, the lubricating oil oxidizes to form acidic products. Furthermore, the engine fuel deposits acidic bodies which find their way into the lubricating oil chamber. These acidic products rapidly deteriorate the lubricating value of the oil. The acidic products thus formed in lubricating oil compositions are severely corrosive to hard metal alloys such as cadmium-silver, copper-lead, etc. This acid corrosion, which may also be called oxidation corrosion, is normally inhibited by the presence of additives which are incorporated in lubricating oil compositions to inhibit the deterioration of the lubricating oil.

Some of the most effective agents which are added to lubricating oils for various purposes contain sulfur. For example, aliphatic polysulfides, metal salts of organo thiophosphates, etc., are highly effective oxidation inhibitors; and sulfurized olefins, xanthate esters and the like are excellent extreme pressure agents. The sulfur contained in these agents is severely corrosive to silver and like metals. This sulfur is not corrosive to such hard-metal alloys as cadmium-silver alloy, which cadmium alloy normally contains less than 1% of silver. However, sulfur readily attacks the pure silver metal wearing surfaces with which this invention is concerned. This attack results in a rapid darkening of the metal surface and the formation of sulfides which subsequently slough oif during engine operation. Thus, there is a considerable loss of metal wearing surface and a pitting of the surfaces.

The corrosion and attack on metals by acidic bodies has been termed oxidation corrosion or acid corrosion, which corrosion is caused by action of acids and peroxides which are formed in lubricating oils as products of oxidation. The corrosion caused by sulfur is herein designated as sulfur corrosion.

In spite of the long desire to lubricate silver bearings with lubricants compounded with active sulfur compounds, it has been impractical to do so in view of the aforementioned severe corrosivity to silver.

Heretofore, in the lubrication of engines having silver metal wearing surfaces, for example, in certain-railroad Diesel engines which use silver wrist pin bushings, it has been diflicult to prevent sulfur attack of the'silver surfaces. Because the silver surfaces (e. g., the silver wrist pin bushings) are susceptible to attack by the sulfur of the sulfur-containing lubricating oil additives, there has been a tendency to eliminate highly effective sulfur-containing additives from the Diesel lubricating oils, thereby losing the remarkable benefits of these additives. As noted hereinabove, certain highly effective lubricating oil anti-oxidants contain sulfur, which is corrosive to silver. These effective anti-oxidants have not been used in Diesel engines containing silver wn'st pin bushings without sulfur attack on the silver. Alternative practices have been to eliminate the sulfur-containing additives from the oil, allowing the oil to deteriorate more rapidly through oxidation; or the sulfur-containing additives have been included in the oil formulation, with the resulting sulfur corrosion of the silver wrist pin bushings. Silver is also used in wearing surfaces in aircraft engines, but because of the presence of the silver, the highly effective sulfur-containing aviation oil additives are not used. Now, however, by using the compounds of this invention, lubricating oils compounded with sulfur-containing additives can be used in Diesel oils, aviation oils, and any other oils where sulfur corrosion of metals is a problem.

Numerous agents have been proposed for inhibiting the sulfur attack on silver; however, the combination of agents set forth in this invention has enhanced activities for reducing the corrosivity to silver by sulfur in lubricating oil compositions. The combination of compounds set forth hereinbelow produces a remarkable synergistic effect in inhibiting sulfur corrosion of silver.

It is, therefore, an object of this invention to provide a non-silver corrosive lubricant containing active sulfur compounds.

It has been ascertained that this object may be attained by the present invention which is concerned with lubricating oil compositions containing active sulfur, which lubricating oil compositions are severely corrosive to silver, and such agents which will inhibit the corrosivity of the sulfur to silver, minimizing the attack on the silver by the sulfur in the lubricant but without adversely affecting the desired functioning of the sulfur compounds per se.

According to the present invention, the combination of Z-thiothiazoles and thiadiazolyl dialkyl dithiocarbamates in lubricating oil compositions inhibits sulfur corrosion of silver. The lubricants of this invention comprise a major portion of oils of lubricating viscosity, and minor amounts each of 2-thio-thiazoles and thiadiazolyl dialkyl dithiocarbamates.

The simplest member of the 2-thio-thiazole group is Z-mercapto-thiazole. Derivatives of Z-mercapto-thiazole, wherein there may be substituents in the ring carbon atoms, are also included herein.

In Z-thio-thiazole, the hydrogen atoms of the thiazole ring may be substituted by short-chained saturated and unsaturated radicals (e. g., methyl, ethyl, propyl, butyl, ethenyl, propenyl, butenyl, etc.). It is preferred that the substitutents contain a total of not more than 20 carbon atoms.

The two adjacent carbon atoms of the Z-thio-thiazoles may form parts of a benzene ring, such. as in Z-mercapto benzothiazole. The 2 mercapto-benzothiazole may be substituted on one or more benzene carbon atoms by short-chained saturated and unsaturated aliphatic radicals (e. g., methyl, ethyl, propyl, butyl, octyl, ethenyl, propenyl, octenyl, etc.). It is preferred that there be not more than four substituent groups on the ring and that these substituents contain a total of not more than 16 carbon atoms.

Examples of Z-thio-thiazoles are: 2-mercapto-benzothiazole; N-cyclohexyl-Z-benzothiazyl sulfonamide, 2- mercapto-S-methyl thiazole; 2-mercapto-4-thiazole; 2- mercapto 4,5 dimethyl thiazole; 2-mercapto-4-ethyl thiazole; 2-mercapto-4,5-diethyl thiazole; phenyl mercapto-benzothiazole; and methyl mercapto-benzothiazole. The thiadiazolyl dithiocarbamates are represented by the following formulas:

wherein the Rs represent straight-chained or branched chained, saturated or unsaturated substantially hydrocarbon (hydrocarbonaceous) radicals selected from the group of alkyl, aralkyl and alkaryl radicals. The alkyl groups present as such or attached to a ring structure each contain from 1 to 20 or more carbon atoms, preferably 2 to 8 carbon atoms. The R groups may or may not be identical.

Examples of the R groups include: ethyl, propyl, propenyl, butyl, butenyl, octyl, octenyl, octadecyl, benzyl, phenyl butyl, phenyl octyl, ethyl phenyl, butyl phenyl, octyl phenyl, etc.

It is preferred to use thiadiazolyl dithiocarbamates wherein the Rs of the above formula represent alkyl radicals.

By essentially hydrocarbon (hydrocarbonaceous) radical is meant those radicals which are composed mainly of hydrogen and carbon, and include such radicals which include, in addition, minor amounts of the substituents such as chlorine, bromine, oxygen, nitrogen, etc.

Examples of thiadiazolyl dithiocarbamates of this invention include:

1,3,4-thiadiazolyl-2,5-bis (diethyl dithiocarbarnate) 1,3,4-thiadiazolyl-2,5-bis(dibutyl dithiocarbamate); 1,3,4-thiadiazolyl-2,S-bis (doctyl dithiocarbamate) 1,3,4-thiadiazolyl-2,5-bis(dioctadecyl dithiocarbamate); 1,2,4-thiadiazolyl-2-thiol-5-diethyl dithiocarbamate; l,3,4-thiadiazolyl-2-thiol-5-dibutyl dithioearbamate; 1,3,4-thiadiazolyl-2-thiol-5-dioctyl dithiocarbamate, etc.

The 2-thio-thiazoles and thiadiazolyl dithiocarbamates of this invention are effective in inhibiting silver corrosion when each is present in amounts as low as about 0.01% (by weight of total composition). Certain of these compounds have only limited solubility in the lubricating oil and are effective at that solubility. When the 2-thio-thiazoles and thiadiazolyl derivatives have sufficient solubility, they are effective when each one is used in amounts as great as 10%. However, it is preferred that the compounds of this invention be used in amounts sufiicient substantially to reduce the corrosion of silver. For example, it is preferred to use the 2-thiothiazole derivatives and the thiadiazolyl compounds each in amounts of from about 0.05% to 0.50%, so that the total is from about 0.10% to about 1.0%. The major proportion of the lubricating oil composition of this invention is the base oil.

It is preferred that the 2-thio-thiazoles and the thiadiazolyl derivatives be used in equal amounts to obtain the greatest benefit therefrom. However, the ratio of 2-thio-thiazoles and thiadiazolyl derivatives can have values varying from 0.33 to 3.

Suitable base oils include a wide variety of lubricating oils such as naphthenic base, paraffin base, and mixed mineral oils, other hydrocarbon lubricants, e. g., lubricating oils derived from coal products, and synthetic oils, e. g., alkylene polymers (such as polymers of propylene, butylenes, etc., and mixtures thereof), alkylene oxide type polymers, dicarboxylic acid esters, liquid esters of acids of phosphorus, alkyl benzene polymers, polymers of silicon, etc. Synthetic oils of the alkylene oxide type polymers which may be used include those exemplified by the alkylene oxide polymers (e. g., propylene oxide polymers) and derivatives, including alkylene oxide polymers prepared by polymerizing alkylene oxides, e. g., propylene oxide, in the presence of water or alcohols, e. g., ethyl alcohol; esters of alkylene oxide type polymers, e. g., acetylated propylene oxide polymers prepared by acetylating propylene oxide polymers containing hydroxyl groups; polyethers prepared from the alkylene glycols (e. g., ethylene glycol), etc. The polymeric products prepared from the various alkylene oxides and alkylene glycols may be polyoxyalkylene diols or polyalkylene glycol derivatives; that is, the terminal hydroxy group can remain as such, or one or both of the terminal hydroxy groups can be removed during the polymerization reaction by etherification or esterification.

Synthetic oils of the dicarboxylic acid ester type include those which are prepared by esterifying such dicarboxylic acids as adipic acid, azelaic acid, suberic acid, sebacic acid, alkenyl succinic acid, fumaric acid, maleic acid, etc., with alcohols such as butyl alcohol, hexyl alcohol, 2-ethyl-hexyl alcohol, dodecyl alcohol, etc. Examples of dicarboxylic acid ester synthetic oils include dibutyl adipate, dihexyl adipate, di-2-ethylhexyl sebacate, di-n-hexyl fumaric polymer, etc.

Synthetic oils of the alkyl benzene type include those which are prepared by alkylating benzene (e. g., dodecyl benzene tetradecyl benzene, etc.)

Synthetic oils of the type of liquid esters of acids of phosphorus include the esters of phosphoric acid, e. g., tricresyl phosphate; the esters of phosphonic acid, e. g., diethyl ester of decane phosphonic acid (or other such esters as obtained by reacting alkyl phosphonyl chlorides with hydroxyl-containing compounds such as phenols and aliphatic alcohols) and with olefin oxides such as propylene oxide, as described in Jensen et 211., U. S. Patent Application 202,396.

Synthetic oils of the type of polymers of silicon include the liquid esters of silicon and the polysiloxanes. The liquid esters of silicon and the polysiloxanes include those exemplified by tetraethyl silicate, tetraisopropyl silicate, tetra(methyl-2-butyl) silicate, tetra(4-methyl- 2-pentyl) silicate, tetra(l-methoxy-Z-propyl) silicate, hexa(4-methyl-2-pentoxy) disiloxane, poly (methyl siloxane), poly(methylphenyl siloxane), etc.

The above base oils may be used individually as such or in various combinations (wherever miscible or whenever made so by the use of metal solvents).

The improved lubricant of the present invention is used wherever elemental sulfur or sulfur compounds containing active sulfur attack metal and are severely corrosive to metals which are being lubricated. Thus, as a result of the present invention, lubricants which normally are severely corrosive to metal parts due to attack by sulfur or active sulfur compounds may now be used without fear of corrosion by incorporating therein the combination of 2-thiothiazole compounds and thiadiazolyl compounds. Severely corrosive is defined as that corrosion which causes a weight loss greater than about 30 mg. in the silver strip test noted hereinbelow.

In order to evaluate the agents of this invention for inhibiting corrosion due to active sulfur, a silver strip corrosion test was made on numerous agents of this invention. This silver strip corrosion test was performed as follows:

A silver metal strip having the dimensions of 2%" x X was first cleaned with a Wire brush until the strip was highly polished. The strip was weighed and the weight recorded. This highly polished silver strip was then placed in a 600 ml. beaker in such a manner that the strip was completely immersed when 300 gm. of the oil being tested was poured into the beaker. The oil was stirred at a temperature of 300 F. for 20 hours, at which time the silver strip was removed and cleaned,

first with chloroform then with petroleum ether. The appearance of the strip was noted. Those strips which had been severely attacked were quite black. The tested silver strips were washed in aqueous solution of potassium cyanide for about 5 minutes toremove the sulfide film adhering to'the strip. After the strip had been washed with potassium cyanide and dried, it was weighed. The difference in weight of the original strip and the strip after the potassium cyanide wash was noted and recorded as the weight loss due to corrosion by the sulfur compound.

The 1,3,4-thiadiazolyl 2,5 bis(dialkyl dithiocarbamates) used in obtaining the dataset forth hereinbelow are manufactured by Sharples Chemicals, Inc., Philadelphia, Pa.

Table I below presents data obtained in the silver strip test" and shows the remarkable effect of the combination of 2-thio-thiazole and thiadiazolyl dithiocarbamates as silver protective agents in reducing the attack on silver by a California solvent refined S. A. E. 30 paraflinic base oil containing a zinc alkyl phenyl dithiophosphate, 2- mercaptobenzothiazole and 1,3,4-thiadiazolyl 2,5-bis- (diethyldithiocarbarnate). The alkyl radical in the zinc alkyl phenyl dithiophosphate contained an average of 14 carbon atoms and was derived from a mixture of propene and butene polymers.

Table II presents data obtained in the silver strip test of a solvent refined S.A.E. 30 parafiin base oil to which was added zinc alkyl phenyl dithiophosphate, 2- mercapto-benzothiazole and 1,3,4-thia-diazolyl 2,5-bis (dibutyl dithiocarbamate).

TABLE II Amount of Amount gt 1,3i4-1tl21i5a(11)ia- Z-mercap o y is benzothiazole (dibutyl'dithiogg l aifi in lubricating carbamate) in Loss'in S oil (Percent lubricating oil g by Weight) (Percent by Weigh t) The eifectiveness of the combination of agents of this invention for inhibiting sulfur corrosion to silver is further shown by the data of Table III. These data were obtained according to a modified ASTM D130-SOT sulfur corrosion test. The silver strips were immersed in the test oils at 300 F. for a period of 3 hours, at which time the strips were removed, solvent-washed free of oil, and inspected visually. The strips were rated as follows: a silver strip which was not discolored was given a rating of zero (0), while a strip which showed pronounced corrosion was given a rating of 5, with proportionate ratings for intermediate discoloration. In these tests, the base oil was a mixture of equal parts by volume of isooctyl hexaoxypropylene acetate and tetra (Z-ethyl hexyl) ortho silicate. Incorporated in the oil was a sulfurcontaining extreme pressure additive commercially marketed by Lubri-Zol Corporation as Anglamol 82.

TABLE HI Amount of Amount of 2-mereapto igi gg g' Visual riffititttlt g g g isfit? car ama e by Weight) in on The action of the silver protective agents of this invention cannot be explained on any simple basis, since their apparent effect appears anomalous. On the one hand, the protective agents prevent destructive attack of the sensitive metal by active sulfur compounds, but on the other hand they do not interfere with the functioning of extreme pressure agents which are believed to act by reaction with the metal surfaces.

As illustrative of various sulfur containing extreme pressure agents, detergents, oxidation inhibitors, etc. which cause oils containing them to be severely corrosive to silver, the following may be mentioned: sulfurized lubricating oils, xanthate esters, chlorinated xanthate esters, dialkyl disulfides, sulfurized fatty oils, metal salts of reaction products of olefins and phosphorus penta sulfide, organic trisulfides, tetra sulfides and penta sulfides (e. g., paraifin wax polysulfide), polyvalent metal salts of organo-substituted acids of phosphorus (e. g., zinc hexyl dithiophosphate), sulfurized olefins (e. g., sulfurized terpenes), xanthic acid derivatives (e. g., dibutyl xanthogen disulfide), etc. The silver protective agents of this invention are also efiective in reducing attack on silver by the sulfur in lubricating oils derived from some of the crude oils characterized by their high sulfur content.

The silver protective agents are especially desirable and preferably used in combination in lubricating oils containing the polyvalent metal salts of organo-substituted thioacids of phosphorus (e. g., zinc alkylphenyl dithiophosphate); however, they can be used in lubricating oils containing such compounds as phenates (e. g.,

calcium cetyl phenates), sulfonates (e. g., calcium petroleum sulfonates) phosphates (e. g., calcium cetyl phosphates) phenols (e. g. 2,6-di-tertiary-butyl-4-methyl phenol), phosphonates (e. g., calcium white oil phosphonate), thiophosphonates (e. g., calcium cetyl thiophosphonates) While the outstanding advantages of these silver protective agents in otherwise severely corrosive lubricants containing sulfur compounds is particularly apparent in heavy duty motor oils, the inventive combination finds application in turbine oils, gear oils, such as for the use in hypoid gears, cutting oils, soluble oils, greases, etc. Thus, the silver protective agents of this invention may be added to lubricating oil compositions containing other agents than sulfur active agents, that is, other agents which are advantageously present as oxidation inhibitors (e. g., phenyl alpha naphthylamine), anti-wear agents (e. g., tricresyl phosphate), rust inhibitors (e. g., metal sulfonates), oiliness agents, blooming agents, viscosity index improvers, pour point depressants, peptizing agents, thickening agents for greases, etc.

The combination of compounds of this invention is also effective in reducing corrosion of silver due to selenium compounds. That is, lubricants which contain active selenium compounds as additives, for example, dialkyl monoselem'des, dialkyl diselenides, dialkyl polyselenides, selenomercaptans, methyl alkyl selenides, etc. show attack on silver by selenium. However, the protective agents of this invention inhibit the attack on silver by lubricants containing selenium compounds.

I claim:

1. A lubricant comprising a major proportion of a silver-corrosive sulfur-containing lubricating oil composition, from 0.05% to 0.5% by weight of Z-mercaptobenzothiazole, and from 0.05% to 0.5 by weight of a thiadiazolyl dithiocarbamate selected from the group consisting of 1,3,4-thiadiazolyl-2,5-bis(diethyl dithio carbamate) and l,3,4-thiadiazolyl-2,5-bis(dibutyl dithiocarbamate).

2. A lubricant comprising a major proportion of an oil of lubricating viscosity having incorporated therein sulfur-containing additives which are corrosive to silver, and which additives inhibit corrosion caused by oxidation, and from 0.05% to 0.5 by weight of Z-mercapto-benzothiazole, and from 0.05% to 0.5 by weight of a thiadiazolyl dithiocarbamate selected from the group consisting of 1,3,4-thiadiazolyl-2,5-bis(diethyl dithiocarbamate) and 1,3,4-thiadiazolyl-2,5-bis(dibutyl dithiocarbamate).

3. A lubricant consisting essentially of a major proportion of an oil of lubricating viscosity having therein sulfur derived from the oil itself, which sulfur is corrosive to silver wearing surfaces, wherein the silver is prescut as substantially pure silver, and 0.05 to 0.5 by weight of 2-mercapto-benzothiazole, and from 0.05 to 0.5 by weight of a thiadiazolyl dithiocarbamate selected from the group consisting of 1,3,4-thiadiazolyl-2,S-bis- (diethyl dithiocarbamate) and 1,3,4-thiadiazolyl-2,5-bis- (dibutyl dithiocarbamate).

4. A lubricant consisting essentially of a major proportion of an oil of lubricating viscosity having incorporated therein sulfur-containing compounds having sulfur corrosive to pure silver wearing surfaces, from 0.05% to 0.5% by weight of Z-mercapto-benzothiazole, and from 0.05% to 0.5% by weight of 1,3,4-thiadiazolyl-2,5-bis- (diethyl dithiocarbamate) 5. A lubricant consisting essentially of a major proportion of an oil of lubricating viscosity having incorporated therein sulfur-containing compounds having sulfur corrosive to pure silver wearing surfaces, from 0.05 to 0.5% by weight of 2-mercapto-benzothiazole, and from 0.05% to 0.5% by weight of 1,3,4-thiadiazolyl-2,S-bis- (dibutyl dithiocarbamate).

References Cited in the file of this patent UNITED STATES PATENTS 

1. A LUBRICANT COMPRISING A MAJOR PROPORTION OF A SILVER-CORROSIVE SULFUR-CONTAINING LUBRICATING OIL COMPOSITION, FROM 0.05% TO 0.5% BY WEIGHT OF 2-MERCAPTOBENZOTHIAZOLE, AND FROM 0.05% TO 0.5% BY WEIGHT OF A THIADIAZOLYL DITHIOCARBAMATE SELECTED FROM THE GROUP CONSISTING OF 1,3,4-THIADIAZOLYL-2,5-BIS(DIBUTYL DITHIOCABMATE) AND 1,3,4-THIADIAZOLYL-2,5-BIS(DIBUTYL DITHIOCARBAMATE). 